Apparatus and method of searching for known sequences

ABSTRACT

Apparatus and method for correlating a received communication of a known sequence over a wireless channel through the use of a finite impulse response (FIR) filter having a small number of taps to reduce hardware requirement by as much as one-half that of conventional techniques while obtaining amplitude degradation which is no worse than experienced when employing conventional techniques.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional application No.60/360,822 filed on Feb. 28, 2002, which is incorporated by reference asif fully set forth.

FIELD OF THE INVENTION

The present invention relates to the required timing resolution versusperformance loss encountered when performing a search for a knowntransmitted signal sequence in a receiver by correlation. The search isperformed in a 3^(rd) generation partnership project (3GPP) widebandcode division multiple access (WCDMA) receiver during a propagation pathsearch or random access channel (RACH) preamble detection.

BACKGROUND

The description of the invention focuses primarily on the frequencydivision duplex (FDD) version of a communication system. The invention,however, is applicable to almost all known sequence search in anycommunication system to search a known sent sequence in a receivedsignal in the time domain.

There are several purposes why a sequence of symbols known to thereceiver might be sent out from a transmitter such as channel estimationwith respect to timing delay, amplitude and phase such as in a pathsearch; signaling for (slotted) ALOHA multiple access collisiondetection and access granting such as with RACH preamble detection; andsignaling of timing relations and even code group allocations, such asin a cell search.

Particularly in cases where lower level signaling is involved, there areusually several different known sequences that possibly can be sent out,and the signaling value is dependent on which one is found. Therefore,the search has to be performed over all available possible, or relevant,sequences. The present invention is applicable whether one sequence issearched for at a time or whether several different searches fordifferent single sequences are performed in parallel or serially.

The exact receive timing of a known sequence is often not known.Unfortunately, this is exactly the parameter of interest, (e.g., forRACH preamble, if the distance and therefore the propagation latencybetween transmitter and receiver are not known). Additionally, thetransmit timing could be completely unknown, such as in cell searching;or the reception of the known sequence could be in different replicaswith respect to timing, amplitude and phase, but these parameters wouldthen be of particular interest, such as in path searching.

In general, there is a certain time window when the sequence is expectedto be received, which is constituted by some transmit timingrelationship, (or simply the repetition rate if the sequence isrepeatedly sent out on a regular basis). Therefore, on the receive side,a search for the sequence is made within the time window, typically byrepeated correlation of the incoming received signal at consecutiveinstances in time followed by a search of maxima or threshold comparisonin the output signal of this correlator. This operation of correlationat consecutive time instances can be viewed as finite impulse response(FIR) filtering of the incoming signal using the expected sequence asthe coefficients for the FIR filter. This is in line with the idea ofusing a matched filter for detection.

In a 3GPP system, the known sequences of symbols are transmitted using apulse shaping filter of the root-raised-cosine (RRC) type. On thereceiver side, an RRC-type filter matched to this transmit pulse isused. The combination of both filters, (in time domain the convolution),is then of the raised-cosine (RC) type. FIG. 1 shows the impulseresponse of an RC filter in time domain, with a filter roll-off factorof 0.22 as used in 3GPP, and being normalized to 1.0 as the maximumamplitude. Amplitude magnitude in dB of the impulse response for thefilter of FIG. 1, is shown in FIG. 2.

Obviously, if the transmit and receive timing for a symbol are fullyaligned, the received signal amplitude is at maximum and for neighboringsymbols spaced at integer multiples of the symbol duration Tc, thereceived signal is zero. This is one of the essential properties ofthese types of filters and is the reason why this type of filter is usedin this application.

If the exact symbol timing is not known, and the reception is off bysome timing offset, then the received signal amplitude is not at maximumany more. With the search of a known sequence with unknown timing, theexact symbol timing will typically not be met. Accordingly, this type oferror almost always occurs.

If the search for a known sequence is performed spaced in time at Tc,then the maximum possible timing error is Tc/2, and the amplitudedegradation resulting from this, as shown in FIG. 2, is about 4 dB,which is prohibitive for performance reasons. For a sequence searchperformed spaced at Tc/2, the maximum timing error is Tc/4, and theamplitude degradation 0.94 dB.

In view of the above, performing the full correlations at a rate of Tc/2is the approach most widely seen in current approaches to the challengeof a known sequence search with unknown timing. However, this approachis not optimum with respect to the processing effort. The problem ofperformance degradation caused by timing mismatch has been solved in theprior art through the use of a simple over-sampling approach conductedat the start of the baseband processing chain. This approach requires asignificant amount of additional hardware as compared with processingthat does not employ over-sampling.

The present invention makes it possible to perform highly hardwaredemanding chip rate processing on a single-sample-per-chip rate asopposed to an over-sampled rate.

In order to cope with the possibility of a timing error, the presentinvention employs an FIR filter structure as an estimation filter whichestimates those samples that have been skipped in the chip rateprocessing. Since the processing is performed on a symbol level and alsosince the FIR filter is very short with respect to its coefficientnumber, the additional hardware required is significantly lower thanthat required for performing over-sampling at the chip rate. Thedegradation of the detection performance is marginal to negligible evenwhen employing FIR filter structures with a low number of taps, suchfilter structures being of simple design and are quite inexpensive toimplement.

Thus, the present invention reduces the processing costs of thecorrelation process by close to 50% while at the same time achievingsimilar performance and at a reduced cost of the necessary hardware ascompared with present day over-sampling techniques employed to deal withtiming mismatch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the impulse response in time domain of an RC filter with aroll-off factor of 0.22.

FIG. 2 is the amplitude magnitudes in dB of the filter of FIG. 1.

FIG. 3 is the convolution of the RC pulse with the filter of the presentinvention.

FIG. 4 is a comparison of the amplitude magnitudes in dB of the overallmaximum attenuation of the present inventive method with the originalcorrelation results and the estimated correlation results.

FIG. 5 is a block diagram of a system for achieving timingsynchronization.

FIG. 6 is a block diagram useful in explaining the “brute force”technique presently being employed.

FIG. 7 is a block diagram useful in explaining the technique of thepresent invention.

FIG. 8 is an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the drawingfigures where like numerals represent like elements throughout.

In the Background Section it was noted that in the search for a knownsequence, when performing the correlation off time, the resultingamplitude can be read out of the RC pulse, dependent on the timing lag.It is assumed that the known sequence has the desired property of havingan autocorrelation function of a single dirac pulse, which is only anapproximation. In reality, this autocorrelation has sidelobes as well,depending on the exact known sequence/scrambling code for which aprecise evaluation would need to be taken into account, but may beneglected herein for simplicity.

Therefore, if correlations against the known sequence spaced in timefrom each other (e.g., at Tc) are performed, then a sampled replica ofthe RC pulse can be seen in the sequence of these correlation results.

In the rare event of exact on-time correlation, this would be at themaximum, and no sidelobes would be visible. In the general case, wheresome timing offset is present, one maximum will be observed and, in theneighboring correlation results, the positive and negative sidelobessampled at Tc according to the RC pulse will be observed.

Since it is desired that the correlation results are calculated at arate of 1/Tc, but it is desired to avoid suffering a 4 dB loss in thecase of a timing offset of Tc/2, the present invention attempts toestimate the missing but desired correlation result values at anadditional timing offset of Tc/2 against the available ones bycollecting and constructively combining the sidelobes together. In thismanner, positive sidelobes will be combined with a positive weight andnegative sidelobes combined with negative weight.

In order to derive the weights more exactly, strong sidelobes can beused to amplify, and weak sidelobes to attenuate, as with maximum ratiocombining theory, (i.e., a matched filter). In the replica of thecorrelation results, which is of the sampled RC type, a FIR filtermatched to this signal is applied, which is then also a sampled RC typeof filter.

For the on-time case and sampling with 1/Tc, the impulse response of theFIR filter is a single dirac pulse, so no further action is necessary.For the Tc/2 shifted case and sampling with 1/Tc, the FIR filter is theRC time pulse sampled at the time instances of Tc=N+½, N being allinteger numbers.

The present invention applies, in addition to a matched filter matchedto a known sequence, which is the correlation filter, a matched filtermatched to the whole known transmission filter chain, which is an RCfilter. This filter, with coefficients like the RC pulse sampled at timeinstances of Tc=N+½, N being all integer numbers, is infinite in lengthand therefore truncation is needed. Assuming truncation of the filter toa length of 4 coefficients a₀ . . . a₃, wherein a₀=a3=RC(t=1.5)=−0.1914and a₁=a₂=RC(t=0.5)=0.6294, (t being normalized to Tc). If computedcorrelation results cr(t) are available at some lags t=0, 1, 2, 3, agood estimate of cr(1.5) can be made using just a 4-tap FIR filter overthe available correlation results: $\begin{matrix}{{c\quad {r(1.5)}} = {\sum\limits_{n = 0}^{3}{a_{n}c\quad {r(n)}}}} & {{{Equation}\quad (1)}\quad}\end{matrix}$

Using this approach it can be estimated from any four surroundingcorrelation results the value of the desired central, not available(because of non-integer but integer+½ Tc timing offset) but desiredcorrelation results to reduce the timing error and resulting amplitudeattenuation.

Since the filter length is truncated, it is an estimation, not an exactcalculation. Also, since the RC pulse has a single-sided frequencybandwidth larger than 1/(2*Tc), but less than 1.22/(2*Tc), more lossresults from the fact that it is undersampled using 1/Tc as the samplerate. Additionally, the bandwidth of the estimation filter used issmaller than 1/(2*Tc). It should be noted that an example where it wouldbe desired to extend the resolution from 2/Tc to 4/Tc, the bandwidthissues would not be relevant. However, since this is not the preferredembodiment application, sampling the RC pulse at 1/Tc rate (i.e.,performing the initial correlation at 1/Tc rate, then estimating theremaining values to get to the 2/Tc rate) is performed in the presentinvention.

Additionally, for the purpose of a sequence search, it is not arequirement to maintain the essential property of the RC type overallpulse shaping filter chain (non-intersymbol interference (ISI)) byzero-crossings at N times Tc for N other than zero). Rather, is itimportant in this application to achieve high peaks for all timingoffsets, such that the peak detection performance is, as far aspossible, independent of the quasi-random timing offset.

As hereinbefore discussed, the present invention preferably utilizes a4-tap FIR filter applied on available computed correlation results attime instances spaced at 1/Tc, to estimate intermediate correlationvalues and thereby increase the timing resolution of the correlationresults to 2/Tc. Any consecutive processing, such as thresholdcomparison or maximum search, is then applied to these correlationresults available at rate of 2/Tc, just as if they had been computed bybrute-force full correlation at rate 2/Tc.

FIG. 5 shows a system model 10 in which a dirac pulse 12 is applied to asequence FIR filter 14 which is applied to a root-raised cosine (RRC)FIR filter 18 forming part of the channel 16. At the receiver end, aroot-raised-cosine (RRC) FIR filter 20 receives the transmitted signal,filter 20 being matched to the transmit pulse. The combination of thefilters 18 and 20, function as a raised-cosine (RC) type filter. A novelaspect of the present invention is the utilization of the known sequencedetector 22 in the signal processing chain. After the interpolation, thepost-processing, e.g., maximum search or threshold detection isperformed at stage 22 in much the same manner as conventional apparatus.Omission of an FIR filter structure from the signal processing chainwould result in a search for the known sequence by correlation to eithersuffer from severe performance degradation or would require the alreadymajor chip rate processing complexity to be doubled.

FIG. 6 shows the “brute force” method wherein the known sequencedetector 22 includes a correlator finite impulse response (FIR) filter24, which receives the incoming signal at the rate of two samples perchip and provides its output to the peak search detector 25, likewiseoperating at the rate of two samples per chip.

By comparison, the novel method of the present invention, shown in FIG.7, provides the incoming signal to the sequence correlator FIR filter 24at the rate of one sample per chip. Its output, also at one sample perchip, is directly applied to multiplexer 28 as well as an estimationfilter 26, which, in the preferred embodiment, is a four (4)-tap FIRfilter.

The signal is applied to FIR filter 24 at the rate of one sample perchip and its output, likewise, at the one sample per chip rate, isprocessed by the estimation FIR filter 26.

Multiplexer 28 receives the two signal streams and alternates passage ofthese streams to the peak search/detector 25 which performs the peaksearch/detection operation at a rate of two samples per chip.

An estimate of the performance of 4-tap FIR filtering for theapplication is set forth below. Since the proposed coefficients for thefilter are taken as the sampled RC pulse itself, for an on-time (i.e. inthis case 3 Tc/2 off the 1/Tc sampling) signal into the filter (assuminga 1.0 peak amplitude), the signal per tap to be multiplied with anassociated coefficient, is identical to the coefficient:

cr(n)=RC(n−1.5)  Equation (2)

The interpolation filter can be considered as a matched filter matchedto the raised-cosine (RC) pulse. Since this pulse is infinite, an idealfilter would also be infinite. By restricting the filter to four (4)taps, further optimization of the coefficients using well known methods,like minimizing the mean square error, are possible. However, the gainedimprovements are not higher than 0.1 dB detection sensitivity.

Using Equation (2) in Equation (1) and the coefficients set forth above,Equation (1), cr(1.5) is estimated as: $\begin{matrix}{{c\quad {r^{\prime}(1.5)}} = {\sum\limits_{n = 0}^{3}\left( a_{n\quad} \right)^{2}}} & {{{Equation}\quad (3)}\quad}\end{matrix}$

In this case, cr′(1.5)=0.8656 is the estimation of the peak at t=1.5,estimated from out of the cr(t) for t=0 . . . 3 (i.e. the 4 surroundingones). This is a loss of −1.25 dB=20 log(0.8656) for the peak of theestimation. The result of Equation (3) represents the energy scalingthat the filter would apply to a white noise signal at it's input. Thismeans that white noise at the input of the filter is attenuated by −0.63dB=10 log(0.8656) to the output.

Since it is desired to obtain a peak for the estimation that isattenuated as little as possible, and at the same time to prevent whitenoise from being either amplified or attenuated, the whole coefficientset of the FIR filter is scaled by1/sqrt(cr′(1.5))=1/sqrt(0.8656)=1.0749. The new coefficient set is thenb₀=b₃=RC(t=1.5)/sqrt(cr′(1.5))=−0.2057 andb₁=b₂=RC(t=0.5)/sqrt(cr′(0.5))=0.6765.

This filter design will not change the energy of a white noise signalwhen passed through the filter. The estimation result with the newscaled filter coefficients, however, will only achieve a value ofcr″(1.5)=sqrt(0.8656)=0.9304. The remaining attenuation on the peak isnow reduced to −0.63 dB=20 log(sqrt(0.8656)). Accordingly, thisattenuation of −0.63 dB is equal to the degradation in signal-to-noiseratio (SNR) at the peak.

It has been demonstrated how much the attenuation is for the newlyscaled estimation filter of the present invention if the true timingoffset from the one correlated at 1/Tc is equal to Tc/2. This case isquite rare, and in general, the timing offset is different andquasi-random. Accordingly, a consideration of the impact of a differenttiming offset on the filter estimation method of the present inventionwill now be given. This is possible if the convolution of the RC pulsewith the filter is observed. The result is shown in FIG. 3.

The difference between the amplitude magnitudes in dB of the overallmaximum attenuation of the present inventive method with the originalcorrelation results together with the estimated correlation results areshown in a common diagram in FIG. 4. As shown in FIG. 4, the maximumattenuation for the method of the present invention is 1.15 dB, which isnot much more than for the brute-force correlation computation performedat rate Tc/2 (0.94 dB there).

The use of a four-tap FIR estimation filter provides performanceequivalent to that of the “brute force” method while yielding areduction of the order of 50% of the hardware utilized to perform the“brute force” method.

Although a larger number of taps may be provided in the estimation FIRfilter 26, the gained improvement in filter performance dropsconsiderably with the inclusion of additional taps. An increase in thenumber of taps however, increases a delay through the filter as well asadding to the complexity of the filter. Thus, the total number of tapsshould preferably be four (4) but could still meaningfully be in a rangeof two (2) to twenty (20). A preferred range is two (2) to ten (10),while the most preferred range is two (2) to four (4).

There are several variants of this interpolation method of the presentinvention to optimize the performance versus the processing effort:

Vary the number of filter taps

Use more than just 1 estimation value spaced at Tc/2 away from truecalculated values spaced at Tc from each other, (e.g. use 2 estimationvalues in between, spaced at Tc/3 and 2-tap filtering).

FIG. 8 shows an arrangement wherein more than one estimation FIR filteris employed. For example, assuming that two estimation filters 26-1 and26-2 are employed, their outputs, together with the output from sequencecorrelation FIR filter 24, are applied to multiplexer 28 ¹, whichdiffers from the multiplexer 28 shown in FIG. 7, in that the outputsfrom 26-1, 26-2 and 22 are fed in sequential fashion to the peak searchdetector 25 which operates at a rate of three times the sample rate. Inthe example given, the estimation FIR filters 26-1 and 26-2 may be two(2)-tap FIR estimation filters. If desired, a greater number ofestimation filters 26 may be employed with the peak search/detector 24operating at a rate of N+1 times the sample rate where N is equal to thenumber of estimation filters employed. It should be noted that thegained performance improvement employing a greater number of estimationfilters likewise drops off quite considerably, the maximum number ofestimation filters 26 should preferably not exceed four (4).

In summary, the present invention proposes using estimations forincreasing the timing resolution of extremely processing-hungrycorrelations over the time domain, with very little extra processingcompared to increasing the resolution in the original correlation.

What is claimed is:
 1. Apparatus for correlating a signal having a known sequence to obtain synchronization, comprising: a sequence correlating finite impulse response (FIR) filter matched to the known sequence; an estimator finite impulse response (FIR) filter matched to the pulse before chip rate processing action on the original from that sequence correlating FIR filter; and a threshold detector acting on the signal from the estimation FIR filter for peak detection; wherein the threshold detector operates at twice a chip rate of the received signal.
 2. Apparatus for correlating a signal having a known sequence to obtain synchronization, comprising: a sequence correlating finite impulse response (FIR) filter matched to the known sequence; an estimator finite impulse response (FIR) filter matched to the pulse before chip rate processing action on the original from that sequence correlating FIR filter; and a threshold detector acting on the signal from the estimation FIR filter for peak detection; wherein said estimator FIR filter comprises several matched filters connected to said sequence correlating filter.
 3. A method for detecting a received signal having a known sequence and a given chip rate, comprising: a) passing the signal through a finite impulse response (FIR) filter for sequence correlation; b) passing the signal obtained at step (a) through n finite impulse response (FIR) estimation filters; and c) sequentially coupling the signals obtained at steps (a) and (b) to a peak search/detector operating at a rate of N+1 times the chip rate.
 4. The method of claim 3 wherein, when n=1 step (b) includes providing a four (4) tap FIR filter.
 5. The method of claim 3 wherein, when n=2, step (b) includes providing first and second two (2) tap FIR filters.
 6. The method of claim 3 wherein step (b) comprises arranging the FIR filters to operate at the chip rate. 